to their physical conditions or concomitants, psychology does not become merged in or subordinated to physiology. Moreover, the fact that our knowledge of the psychic line is broken does not prove that the line itself is broken. More careful observation may lead to the discovery of the missing links. And where observation leaves us in the lurch, we can have recourse to hypotheses, and here it is to be noted that the physiological hypothesis is not the only possibility. Besides, our knowledge of the physiological chain is not continuous either; here too there are gaps, and here too the gaps are bridged over by theory. The physiologist simply assumes continuity; his fundamental hypothesis is that there can be no gaps in the material world. Formerly he had recourse to animal spirits, vital force, and soul to fill out the gaps, and even to-day many scientists refuse to rest content with the purely mechanical theory of the world. Finally, if there is not a certain amount of discoverable uniformity on the mental side, the physiologist has no clue to the study of the brain processes upon which the phenomena are said to depend. If there is no coherence or order in the effects, how can there be coherence or order among the causes? If psychology is impossible because there is no law on the mental side, then cerebral physiology is impossible because there is no law on the physiological side, and also because we have no key with which to open the secrets of the brain.
The argument is often made in favor of affiliating psychology with natural science on the score of method. Psychology, it is held, must investigate its facts as the natural sciences investigate theirs, by the methods of observation and experiment. It must also measure its phenomena or apply the method of numerical determination wherever this is possible. The methods of observation, experiment, and measurement are the methods of science, their employment is what makes a science exact, and presupposes thorough scientific training on the part of those who use them. Psychology is therefore a natural science and belongs in that field.
This reasoning does not seem to me to hold good. True, the general method of psychology is the same as that of every other
